It is proposed to study two complementary specificities of human gamma/delta T-cells. Phenyl pyrophosphates and related molecules stimulate large numbers of human V-gamma-2-/V-delta-2 T-cells. Based on the hypothesis that this response is triggered by binding interactions between the gamma/delta TCR and the phosphate antigens (PA) resembling those of immunoglobulins and haptens, the reactive gamma/delta TCR will be mutated to determine the antigen combining site for isopentenyl pyrophosphate, a naturally occurring PA. Direct binding studies will be carried out using soluble gamma/delta TCR constructs and PA. The biological significance of the PA reactivity of gamma/delta T-cells will be studied in nonhuman primates, by immunizing with these antigens and assessing protective effects, and by monitoring the consequences of systemic gamma/delta T- cell depletion in mycobacterial infections. V-delta T-cells recognize CD1. CD1 recognition by gamma/delta T-cells will be studied as an example of TCR alpha beta-like ligand recognition. CD1c molecules can be recognized without addition of foreign antigens. Gamma/delta T-cell clones responding to CD1c in the absence of added antigens will be characterized phenotypically and functionally, and the possibility that endogenous antigens are required for CD1c recognition will be tested. Secondly, a newly discovered gamma/delta T-cell response to mycobacterial antigens in the presence of CD1 will be investigated. Again, reactive cells will be characterized. In addition, the molecular nature of the stimulatory mycobacterial antigen(s) in this response, predictably quite different from the PA stimulating the responses of V- gamma-2/V-delta-2 cells, will be determined. Studies that define ligand specificities of gamma/delta T-cells are essential to understanding the biological role of this ubiquitous but still enigmatic class of lymphocytes.